Impaired insulin action, or insulin resistance, is a hallmark of Non-Insulin-Dependent Diabetes Mellitus (NIDDM). Because insulin resistance is a major factor in the pathogenesis of NIDDM, understanding the mechanisms of insulin resistance has potential implications in identifying novel means to improve insulin sensitivity in individuals predisposed to NIDDM. Insulin binding to its receptor activates the tyrosine kinase of the receptor to cause phosphorylation of the receptor and of other substrates, such as ppl20, a plasma membrane glycoprotein in the hepatocyte. pp120 is phosphorylated on Ser503 in the intracellular domain by cAMP-dependent kinase in the absence of insulin, and this phosphorylation is required for its phosphorylation on Tyr488 by the insulin receptor kinase in response to insulin. The role of ppl20 in insulin action is not well understood. pp120 expression in cultured cells was correlated with increased rate of insulin clearance from the medium through a mechanism of receptor-mediated endocytosis, suggesting that pp120 is important in the process of insulin clearance from the portal circulation. In contrast, expression of phosphorylation-defective pp120 isoforms (truncated and the Y488F and S503A site-directed mutants) did not increase receptor-mediated insulin internalization, suggesting that the effect of p120 on insulin endocytosis depends on its phosphorylation state. Immunofluorescence and biotin-labeling studies suggested that pp120 exerts its effect by undergoing receptor-mediated internalization in response to insulin. Thus, it appears that pp120 takes part in a complex of proteins that target the insulin receptor to endocytosis vesicles. The complex formation between pp120, at Tyr488, and the insulin receptor, at Tyr960 of its juxtamembrane domain, appears to be mediated by intracellular proteins. We herein propose to identify these proteins. Additionally, we propose to address the role of ppl20 in the mechanism of insulin action in vivo. To this end, we have generated a transgenic mouse overexpressing a phosphorylation-defective S503A isoform of ppl20 in liver. The transgenic line will address whether expression of a phosphorylation-defective pp 120 is associated with a blunted ability to remove excess insulin from the portal circulation, causing peripheral hyperinsulinemia. Since hyperinsulinemia leads to receptor down-regulation on target tissues, it is usually associated with insulin resistance. These proposed studies should provide novel insights into a potential mechanism of hyperinsulinemia, insulin resistance and diabetes.